Imaging Sample Acidification Triggered by Electrochemically Activated Polyaniline

Abstract

In this letter, we demonstrate 2D acidification of samples at environmental and physiological pH with an electrochemically activated polyaniline (PANI) mesh. A novel sensor–actuator concept is conceived for such a purpose. The sample is sandwiched between the PANI (actuator) and a planar pH optode (sensor) placed at a very close distance (∼0.50 mm). Upon application of a mild potential to the mesh, in contrast to previously reported acidification approaches, PANI releases a significant number of protons, causing an acid–base titration in the sample. This process is monitored in time and space by the pH optode, providing chemical imaging of the pH decrease along the dynamic titration via photographic acquisition. Acidification of samples at varying buffer capacity has been investigated: the higher the buffer capacity, the more time (and therefore proton charge) was needed to reach a pH of 4.5 or even lower. Also, the ability to map spatial differences in buffer capacity within a sample during the acid–base titration was unprecedentedly proven. The sensor–actuator concept could be used for monitoring certain analytes in samples that specifically require acidification pretreatment. Particularly, in combination with different optodes, dynamic mapping of concentration gradients will be accessible in complex environmental samples ranging from roots and sediments to bacterial aggregates.

Imaging of CO2 and Dissolved Inorganic Carbon via Electrochemical Acidification–Optode Tandem

“…The PANI–optode tandem needs to be calibrated only once for each optode batch (i.e., at the beginning of its utilization). The PANI film is activated once per sample imaging, and then, it is regenerated before being used for the next sample (10 mM H 2 SO 4 at 0 V versus the Ag/AgCl RE, Supporting Information). With this regeneration procedure, the PANI mesh can be re-used for more than 70 activations over a minimum of two weeks. , Because of the regeneration step, the principle is not suitable for continuous but for discrete dynamic and/or long-term measurements of DIC along the lifetime of the PANI–optode system.…”

“…The PANI mesh synthesis was based on a previously published protocol (see Figure S2). , For sample acidification, the PANI mesh was activated by applying a constant potential of 0.4 V with respect to the open circuit potential (OCP) for 600 s. This process was previously demonstrated to decrease the sample pH to levels of <4.5 . For more information about the PANI synthesis, experimental setup, equipment, chemicals, and calibration protocol, the reader is referred to the Supporting Information.…”

“…The concept was also used to map buffer capacity gradients in heterogeneous samples. 21 Effectively, a PANI mesh can homogeneously shift the pH below 4.5 in samples at environmental buffer concentrations (<10 mM bicarbonate equivalents), which corresponds to 99.9% conversion of all the dissolved inorganic carbonate species to CO 2 . 22 In this work, we demonstrate the use of a PANI mesh electrode to locally acidify a sample in contact with a planar CO 2 optode−PANI architecture, which allows DIC to be characterized with high spatial resolution.…”

“…Moreover, in a recent letter, the combination of a PANI mesh electrode with a planar pH optode has allowed for the 2D visualization of PANI-mediated electrochemical acidification in diverse samples. The concept was also used to map buffer capacity gradients in heterogeneous samples . Effectively, a PANI mesh can homogeneously shift the pH below 4.5 in samples at environmental buffer concentrations (<10 mM bicarbonate equivalents), which corresponds to 99.9% conversion of all the dissolved inorganic carbonate species to CO 2 …”

Optical sensors (optodes) for multiparameter chemical imaging: classification, challenges, and prospects

“…Recently, Steininger et al demonstrated that an electroactive polymeric mesh can be placed in front of an optode. 67 This mesh was coated with conductive polymer polyaniline (PANI), known as a proton pump. 68 Protons can be released from the polymer by simply applying a mild potential that alters the pH locally in front of the optode.…”